Since the advent of a resist for KrF excimer laser (248 nm), an image forming method called chemical amplification is used as an image forming method for a resist so as to compensate for sensitivity reduction caused by light absorption. For example, the image forming method by positive tone chemical amplification is an image forming method of decomposing an acid generator in the exposed area upon exposure to produce an acid, converting an alkali-insoluble group into an alkali-soluble group by using the generated acid as a reaction catalyst in the baking after exposure (PEB: Post Exposure bake), and removing the exposed area by alkali development.
Along with miniaturization of a semiconductor device, the trend is moving into a shorter wavelength of the exposure light source and a higher numerical aperture (high NA) of the projection lens, and an exposure machine using an ArF excimer laser with a wavelength of 193 nm as a light source has been so far developed. As commonly well known, these factors can be expressed by the following formulae:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1 and k2 are coefficients related to the process.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally advocated as a technique for raising the resolution.
As for the “effect of immersion”, assuming that NA0=sin θ, the above-described resolution and depth of focus in the immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of the beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
A double exposure technology and a double patterning technology are being advocated as a technique for more enhancing the resolution. These technologies are to make small k1 in the above-described formula of resolution and are positioned as a resolution-increasing technique.
In the conventional pattern formation of an electronic device such as semiconductor device, a mask or reticle pattern in a size of 4 to 5 times larger than the pattern intended to form is reduced and transferred on an exposure target such as wafer by using a reduction projection exposure apparatus.
However, the dimensional miniaturization brings about a problem that in the conventional exposure system, lights irradiated on adjacent patterns interfere with each other to decrease the optical contrast. Therefore, in such a technique, it is devised to divide the exposure mask design into two or more parts and synthesize an image by independently exposing respective masks. In this double exposure system where the exposure mask design is divided, the image of the design must be again synthesized on an exposure target (wafer) and therefore, the division of the mask design must be devised so that the pattern on the reticle can be faithfully reproduced on the exposure target.
Studies on applying the effect of these double exposure systems to the transfer of a fine image pattern of a semiconductor device are introduced, for example, in Patent Document 1.
Also, the recent progress of the double exposure technology is reported, for example, in Non-Patent Documents 1, 2 and 3.
However, when the pattern formation is performed by merely applying a conventional resist composition to a conventional resist process, there arises a problem that sufficient exposure margin or depth of focus cannot be obtained, because in these double exposure systems, the pattern formation needs to be performed in the vicinity of resolution limit of the resist.
In other words, if a pattern forming process as described, for example, in Patent Document 2 where a resist composition containing a resin capable of increasing the polarity upon exposure is applied on a substrate and the resist film is exposed and developed to dissolve the exposed area with an alkali developer, or a pattern forming process as described, for example, in Patent Document 3 where a resist composition containing a resin capable of increasing the molecular weight upon exposure is applied on a substrate and the resist film is exposed and developed to dissolve the unexposed area with an alkali developer, is applied to the double exposure process, a sufficiently high resolving performance cannot be obtained.
With respect to the developer for g-line, i-line, KrF, ArF, EB or EUV lithography, an aqueous alkali developer of 2.38 mass % TMAH (tetramethylammonium hydroxide) is being used at present for general purposes.
Other than the above-described developer, for example, Patent Document 4 describes a developer for developing a resist material containing a copolymer of a styrene-based monomer and an acryl-based monomer and dissolving the exposed portion, where the developer contains an aliphatic linear ether-based solvent or aromatic ether-based solvent and a ketone-based solvent having a carbon number of 5 or more; Patent Document 5 describes a developer for developing a resist material whose molecular weight is decreased as a result of breakage of the polymer chain upon irradiation with radiation, thereby dissolving the exposed portion, where the developer contains at least two or more acetic acid groups, ketone groups, ether groups or phenyl groups and has a molecular weight of 150 or more; and Patent Document 6 describes a developer for developing the unexposed portion of a resist material mainly composed of a photosensitive polyhydroxy ether resin obtained by the reaction of a polyhydroxy ether resin with a glycidyl (meth)acrylate, where the developer is an aromatic compound having a carbon number of 6 to 12 or a mixed solvent containing 50 mass % or more of an aromatic compound having a carbon number of 6 to 12.
Forming a pattern with overall good performance is of course preferred, but it is actually very difficult to find out an appropriate combination of a resist composition, a developer, a rinsing solution and the like required therefor, and improvements are being demanded. In particular, the resolved line width of the resist becomes finer, and this involves requirements to improve the profile such as pattern undercut, improve the line edge roughness performance of line pattern, and further improve the in-plane uniformity of pattern dimension.
Furthermore, the thing that is provided by the above-described combinations of a resist composition and a developer is only a system of forming a pattern by combining a specific resist composition with a high-polarity alkali developer or a developer containing a low-polarity organic solvent. That is, as shown in FIG. 1, in the case of a positive tone system (a combination of a resist composition and a positive tone developer), a material for performing pattern formation by selectively dissolving and removing a region having strong light irradiation intensity out of the spatial frequency of an optical image is merely provided. On the other hand, as for the combination of a negative tone system (a resist composition and a negative tone developer), a material system for performing pattern formation by selectively dissolving and removing a region having a weak light irradiation intensity is merely provided.
The term “positive tone developer” as used herein indicates a developer that selectively dissolves and removes the exposed area not lower than a predetermined threshold value shown by a solid line in FIG. 1, and the “negative tone developer” indicates a developer that selectively dissolves and removes the exposed area not higher than the predetermined threshold value. A development step using a positive tone developer is called positive tone development (sometimes referred to as a positive tone development step), and a development step using a negative tone developer is called negative tone development (sometimes referred to as a negative tone development step).
On the other hand, a double developing technique as a double patterning technology for enhancing the resolution is described in Patent Document 7. In this example, an image forming method by chemical amplification in general is utilized, and by making use of a property that the polarity of a resin in a resist composition when exposed becomes a high polarity in a high light intensity region and becomes a low polarity in a low light intensity region, a high exposure region of a specific resist film is dissolved with a high-polarity developer, thereby effecting positive tone development, while a low exposure region is dissolved with a low-polarity developer, thereby effecting negative tone development. More specifically, as shown in FIG. 2, a region not lower than an exposure dose E2 of irradiated light 1 is dissolved using an aqueous alkali solution as the positive tone developer, and a region not higher than an exposure dose E1 is dissolved using a specific organic solvent as the negative tone developer, whereby, as shown in FIG. 2, a region with a medium exposure dose (between E2 and E1) is allowed to remain without being developed and an L/S pattern 3 having a pitch half the pitch of the exposure mask 2 is formed on a wafer 4.
However, it is very difficult to select an optimal combination of a resist composition and a negative tone developer, and in the example above, there is a problem that developability when using a negative tone developer is bad.
Furthermore, in forming a fine pattern by double development, merely giving good resolution on use of a negative tone or positive tone developer alone is insufficient, but the resist is required to exhibit good pattern resolution for both a negative tone developer and a positive tone developer.    Patent Document 1: JP-A-2006-156422 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    Patent Document 2: JP-A-2001-109154    Patent Document 3: JP-A-2003-76019    Patent Document 4: JP-A-2001-215731    Patent Document 5: JP-A-2006-227174    Patent Document 6: JP-A-6-194847    Patent Document 7: JP-A-2000-199953    Non-Patent Document 1: SPIE Proc 5754, 1508 (2005)    Non-Patent Document 2: SPIE Proc 5377, 1315 (2004)    Non-Patent Document 3: SPIE Proc 61531k-1 (2006)